1. Field of the Invention
The present invention relates to a toner for use in electrophotography. In addition, the present invention also relates to a developer, an image forming method, an image forming apparatus, a process cartridge, and a toner container.
2. Discussion of the Background
In electrophotography, an image is typically formed as follows:    (1) an electrostatic latent image is formed on a photoreceptor (i.e., an image bearing member);    (2) the electrostatic latent image is developed with a developer to form a visible image (i.e., a toner image);    (3) the visible image is transferred onto a recording medium such as paper; and    (4) the transferred image is fixed on the recording medium upon application of heat, pressure, solvent vapor, and/or the like thereto.This method is disclosed in, for example, U.S. Pat. No. 2,297,691.
Developers used for electrophotography are classified into one-component developers consisting essentially of a magnetic toner or a non-magnetic toner, and two-component developers consisting essentially of a toner and a carrier. One-component developing methods, for which a one-component developer is used, are classified into magnetic one-component developing methods, in which a toner is held on a developing roller due to magnetic force, and non-magnetic one-component developing methods.
A toner is typically manufactured by a kneading-pulverization method in which a thermoplastic resin is melt-kneaded together with other toner constituents (such as colorants), followed by pulverization and classification. The thus prepared toner (hereinafter referred to as pulverization toner) is optionally mixed with a particulate inorganic or organic material to improve fluidity and cleanability thereof.
A pulverization toner is typically fixed on a recording medium upon application of heat thereto using a heat roller. When the temperature of the heat roller is too high, offset problem tends to be caused in that part of a fused toner is adhered to the surface of the heat roller. In contrast, when the temperature of the heat roller is too low, the toner cannot be sufficiently fused. Recently, demands for energy saving and downsizing of the apparatus have increased, and therefore a need exists for a toner which minimizes hot offset (this property is hereinafter referred to as hot offset resistance) and which can be fixed at low temperatures (this property is hereinafter referred to as low temperature fixability). Since full-color copiers and printers are required to produce images having good glossiness and color reproducibility, toners having a low melting point are preferably used therein. However, since such toners have poor hot offset resistance and poor thermostable preservability under high temperature and high humidity conditions, a fixing oil (such as silicone oil) is applied to the heat roller of the full-color machines to improve the releasability thereof. In this case, the machine needs an oil tank, a fixing oil applying system, and the like, and therefore the full-color machine must be larger and the fixing system becomes complicated. In addition, the heat roller is easily damaged, and therefore maintenance has to be constantly performed. There is another problem such that the oil applied to the heat roller tends to adhere to copier papers and overhead projection (OHP) films, resulting in deterioration of the color tone of the produced images.
In attempting to solve these problems, a technique in which a release agent (such as wax) is added to a toner is proposed and widely used to prevent the toner from adhering to the heat roller without applying an oil thereto. Releasability of the toner greatly depends upon dispersing conditions of the wax in the toner. When the wax is compatible with the binder resin used, the toner has no releasability. When the wax is incompatible with the binder resin and forms domains thereof in the toner, the toner has releasability. In this case, when the domains are too large, the amount of the wax existing near the surface of the toner relatively increases. Thereby, the toner particles tend to aggregate, resulting in deterioration of fluidity thereof. In addition, the wax tends to form films thereof on a carrier, a photoreceptor, and the like, after a long period of use, and therefore the image quality deteriorates. When the domains are too small, the wax is too excessively dispersed to impart good releasability to the toner.
It is difficult to control the size of the wax domain in pulverization toners. In addition, since the wax tends to exist at pulverized sections, i.e., the surface of the toner particles, the toner has poor fluidity and the wax forms films thereof on the other image forming members, as mentioned above. Pulverization toners have another drawback of typically having a broad particle diameter distribution. As a result, the toner cannot be uniformly friction-charged and tends to cause background fouling in that the background portion of an image is soiled with toner particles. It is difficult to obtain a pulverized toner having a volume average particle diameter of from 2 to 8 μm in terms of manufacturing efficiency. Because of these reasons, pulverized toners cannot satisfy the demands for producing high quality images.
On the other hand, toners manufactured in an aqueous medium have received attention recently. Because such toners have a narrow particle diameter distribution and a small particle diameter, high quality and high definition images can be produced. A release agent (such as wax) can be well dispersed therein, resulting in impartment of good hot offset resistance and low temperature fixability to the toner. The toner also has a uniform chargeability, and therefore transferability improves. In addition, because of having high fluidity, the toner has advantages in designing the developing system such that various hoppers can be used and the torque for rotating the developing roller can be decreased.
As toners manufactured in an aqueous medium (hereinafter referred to as chemical toners), suspension polymerization toners, emulsion aggregation toners, and the like are known.
In a suspension polymerization method, toner constituents such as a monomer, a polymerization initiator, a colorant, and a release agent are added to an aqueous medium containing a dispersing agent to form oil droplets, and then the oil droplets are heated so that the monomer therein is subjected to a polymerization reaction. The suspension polymerization method has an advantage of producing a toner having a small particle diameter. However, the suspension polymerization method has a drawback such that a dispersing agent, which tends to deteriorate chargeability of the resultant toner, is needed. When the aqueous medium contains no dispersing agent, the release agent tends to exist deep inside of the oil droplets, and therefore the resultant toner cannot have an adequate amount of the release agent on the surface thereof.
In the emulsion aggregation method, toner particles are prepared as follows:    (1) a binder resin (e.g., a polyester resin), which is dissolved in a solvent, is dispersed (emulsified) in an aqueous medium, and then the solvent is removed therefrom to prepare a dispersion of fine particles of the binder resin;    (2) the dispersion of fine particles of the binder resin are mixed with an aqueous dispersion of other toner constituents (such as a colorant, a release agent (e.g., a wax), and the like), so that fine particles of the binder resin and the toner constituents aggregate; and    (3) the aggregated particles are heated to be fused, to prepare toner particles.This method is disclosed in, for example, Japanese Patent No. (hereinafter referred to as JP) 3577390 and published unexamined Japanese Patent Application No. (hereinafter referred to as JP-A) 11-007156.
This method has an advantage of producing a toner having a sharp particle diameter distribution without performing classification, because ultra-fine toner particles are not produced, i.e., the emulsification is performed efficiently. However, if the fine particles of the binder resin are aggregated without application of heat, the fine particles cannot sufficiently be united with each other, resulting in the occurrence of fracture at interfaces between the particles constituting the resultant toner particles. Therefore, it is necessary to aggregate the fine particles upon application of heat. However, when the aggregated particles are heated, the wax tends to come out to the surface of the aggregated particles, and each of the dispersed wax particles tends to aggregate. As a result, the wax cannot be appropriately dispersed in the resultant toner. In particular, a release agent having a low melting point easily exudes from the aggregated particles when being heated. A toner including such a release agent has poor releasability, and therefore such a toner is not suitable for use in oilless heat roll fixing methods.
JP-A 2004-226669 discloses a toner, on a surface of which release agent particles which are covered with a vinyl polymer or into which a vinyl polymer penetrates are uniformly and firmly adhered, wherein the release agent particles are prepared by polymerizing a vinyl monomer using a water-soluble polymerization initiator in an emulsion of the release agent. The above release agent particles are added in an aqueous medium in which a toner constituent mixture is emulsified. In this method, it is necessary to polymerize the vinyl monomer. Since the vinyl polymer included in the release agent particles has a high glass transition temperature (Tg), there is a problem such that the resultant toner has poor releasability and low temperature fixability.
JP 2663016 discloses a toner obtained by subjecting a monomer liquid containing a material having a polar group and a release agent to a suspension polymerization. It is described therein that a wax having a low melting point, which cannot be used for the pulverization method, can be used for this method. It is also described therein that nonpolar components such as release agents tend not to exist near the surface of the toner particles whereas polar components tend to exist near the surface of the toner particles, and therefore the resultant toner has a pseudo-capsule structure. However, no mention is made of the real dispersing condition of the wax in the toner.
JP 3225889 discloses a toner including a wax in an amount of from 0.1 to 40% by weight, and at a surface of which the wax exists in an amount of from 1 to 10% by weight, based on the total amount of toner constituents existing at the surface of the toner. The amount of the wax existing at the surface of the toner is determined by ESCA (electron spectroscopy for chemical analysis). However, since the analyzable depth of ESCA is about 0.1 μm (i.e., only a surface region having a depth of 0.1 μm from the outermost surface of the toner can be analyzed with ESCA), the dispersing conditions of the wax existing deep inside of the toner are unknown.
JP-A 2002-6541 discloses a toner including wax particles which exist inside the toner particles while locally existing on the surface of the toner particles. However, no mention is made of detailed dispersing conditions of the wax particles existing near the surface of the toner.
JP-A 2004-246345 discloses a toner, on a surface of which a specific amount of wax exists. The amount of the wax existing on the surface of the toner is determined by FTIR-ATR (Fourier transform infrared spectroscopy attenuated total reflectance). However, it is difficult to improve fixability of the toner only by controlling the dispersing condition of the wax, while imparting a good combination of toner blocking resistance, hot offset resistance, toner filming resistance, and resistance to a paper winding problem such that a receiving paper sheet having a toner image thereon is wound round a fixing member due to adhesion of the toner image to the fixing member.
Because of these reasons, a need exists for a toner manufacturing method which can stably and efficiently produce a toner having a good combination of low temperature fixability, toner filming resistance, and thermostable preservability, and which can produce high quality images, while having advantages of the chemical toners such as small particle diameter, narrow particle diameter distribution, and high fluidity.
In typical fixing processes, heat pressure fixing methods are preferably used in which an unfixed toner image is melted upon application of heat and pressure by directly contacting a fixing member (such as a fixing roller and a fixing belt), and then fixed on a recording material (such as a paper). The heat pressure fixing methods have advantages in terms of thermal efficiency, simplicity of the fixing mechanism, and manufacturing cost of the fixing member.
JP-A 11-329700 discloses a belt fixing device adopting electromagnetic induction heating. The fixing device includes a fixing roller, a facing roller consisting of a non-magnetic material and arranged in parallel with the fixing roller, an endless fixing belt tightly stretched with the fixing roller and the facing roller, an induction coil configured to externally heat the fixing belt, and a pressing roller configured to press the fixing roller with the fixing belt therebetween. A recording paper having a toner image thereon passes through a nip formed between the fixing belt and the pressing roller so that the toner image is fixed on the recording paper by the heat of the fixing belt and the pressure of the pressing roller.
FIG. 1 is a schematic view illustrating the cross section of an embodiment of a typical fixing belt. The fixing belt includes a substrate 1, an exothermic layer 2, an elastic layer 3, and a release layer 4, wherein the layers 2, 3, and 4 are overlaid on the substrate 1 in this order.
The substrate consists of an endless belt made of a thermostable resin. Specific examples of the thermostable resins include, but are not limited to, polyimides, polyamideimides, polyetheretherketones (PEEK), etc. The substrate 1 typically has a thickness of from 20 to 100 μm in view of stiffness and thermal capacity thereof.
The exothermic layer 2 consists of a metal such as SUS, iron, nickel, manganese, titanium, chromium, and copper. The elastic layer 3 is necessary for improving uniformity of the produced images, and consists of a thermostable rubber, such as silicone rubbers and fluorocarbon rubbers, having a thickness of from 100 to 300 μm. The release layer 4 consists of a resin having good thermostability and durability such as fluorocarbon resins, because the release layer 4 contacts a transfer paper and a toner image under pressure.
In the fixing device disclosed in JP-A 11-329700, the fixing belt is merely heated with the induction coil while the temperature of the fixing belt is not controlled, and thereby hot offset tends to occur at both ends of the fixing belt. This is because when small-sized recording papers continuously pass through the fixing belt, the papers draw heat only from the central part of the fixing belt, and therefore the fixing belt is heated to raise the temperature of the central part. In this case, the temperature of both ends of the fixing belt excessively increases. As a result, hot offset tends to occur only at both ends of the fixing belt when large-sized papers pass through the fixing belt under such a condition.
In conventional fixing devices such as the fixing device disclosed in JP-A 11-329700, the facing roller contains bearings, which have large thermal capacity, at both ends. Therefore, although the fixing belt is heated with the induction coil, the heat diffuses into both ends (i.e., bearings) of the facing belt. As a result, the temperature rising speed of both ends of the facing belt is slower than that of the central part of the facing belt, as shown in FIG. 2. It takes a long time to start up such a fixing device.
JP-A 2002-268436 discloses a fixing device including an endless fixing belt which is tightly stretched with a fixing roller and a heat roller so as to have a small curvature radius at a fixing nip. The fixing belt endlessly moves while being heated with the heat roller and contacts a toner image formed on a transfer material upon application of pressure to fix the toner image thereon. The fixing belt includes a substrate consisting of a thermostable resin (such as polyimides) or a metal, an elastic layer consisting of a thermostable rubber or an elastomer, and a release layer serving as an outermost layer and consisting of a fluorocarbon resin. The release layer is formed by covering the elastic layer with a fluorocarbon resin tube which is prepared by extrusion, and then subjecting the fluorocarbon resin to heat treatment. The release layer can also be formed by applying a particulate fluorocarbon resin to the elastic layer using a spray and the like, and then subjecting the fluorocarbon resin to a heat treatment. A fixing belt having a release layer consisting of a fluorocarbon resin has good releasability and thermostability. In particular, such a fixing belt has great releasability, and therefore hot offset and paper winding problems hardly occur. However, fluorocarbon resins have poor flexibility. Therefore, when the fixing belt has a small curvature radius, cracks tend to appear on the release layer after long repeated use, resulting in deterioration of durability of the fixing belt.
Various attempts have been made to solve these problems. For example, a presentation entitled “A Study on On-Demand Fusing Technology (A-11)” was made at Japan Hardcopy '94 (The Annual Conference of the Society of Electrophotography of Japan, held on Jun. 23 and 24, 1994). However these attempts are not sufficient to solve the above problems.
Because of these reasons, a need exists for a fixing device which can produce a high quality images for a long period of time.